One of the most prevailing ideas about learning and memory is that these capabilities are attributes that distinguish the 'more recent' or 'more evolved' vertebrates, such as mammals and birds. According to this customary view, fishes represent the 'most primitive' and 'least evolved' vertebrate group and are supposed to have developed only relatively simple neural circuits, sustaining elemental forms of behaviour. In contrast, other vertebrates such as mammals would be characterized by higher cognitive capacity and behavioural flexibility, mainly associated with the expansion of the telencephalon and the emergence of the six-layered neocortex (Papez 1929; Romer 1962; Jerison 1973; MacLean 1990). Thus, the behaviour of fishes is thought to be mainly 'mechanical' or 'reflex' and lacking a significant degree of learning and memory capabilities. This pervasive idea, which is a consequence of the hybridizing of some Darwinian concepts with the traditional Aristotelian idea of scala naturae, has dominated the landscape of comparative psychology and neuroscience during the past century (Hodos & Campbell 1969, 1990; Deacon 1990). The earlier established theories about vertebrate brain evolution (Papez 1929; Ariens-Kappers et al. 1936; Crosby & Schnitzlein 1983; MacLean 1990) considered the forebrain of actinopterygian fishes to consist mainly of a subpallium ('paleostriatum') and a very small pallium ('paleocortex'), both entirely dominated by olfactory inputs and consequently lacking a significant role, if any, in complex cognitive capabilities. According to these theories, the fish telencephalon should lack an 'archistriatum', a 'neostriatum', and an 'archicortex' (the proposed antecedents of the mammalian pallial amygdala, caudate and putamen, and hippocampus, respectively), and of course it should also lack a 'neocortex'. All these structures would have evolved later, supporting cognition and intelligent behaviour in more recent and complex vertebrate groups.